<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    * {
      box-sizing: border-box;
    }

    div {
      width: 0;
      height: 0;
      background-color: red;
      /* 
      边框 
      1 solid 实线 
      2 dashed 虚线
      */
      /* bd */
      /* border: 1px dashed #000; */

      /* border-top: 10px solid #fff;
      border-right: 10px solid #fff;
      border-bottom: 10px solid green;
      border-left: 10px solid green; */



      width: 100px;
      height: 50px;
      border-radius: 25px;


    }
  </style>
</head>

<body>
  <div></div>
</body>

</html>